O Clavis David

This means:  O Key of David.  It is a title for the Messiah from the Old Testament.  It is the O Antiphon for today.  Here is the Full Antiphon.  Use it in your prayer today:

O Key of David and sceptre of the House of Israel;
you open and no one can shut;
you shut and no one can open:
Come and lead the prisoners from the prison house,
those who dwell in darkness and the shadow of death.

peace